OFFSET
0,2
COMMENTS
Positive values of y in the Diophantine equation 21*x+1 = y^2; the corresponding values of x are given in A219391.
Equivalently, numbers that are congruent to {1,8,13,20} mod 21.
The product of any two terms belongs to the sequence and therefore also a(n)^2, a(n)^3, a(n)^4 etc.
LINKS
Bruno Berselli, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (1,0,0,1,-1).
FORMULA
G.f.: (1+7*x+5*x^2+7*x^3+x^4)/((1+x)*(1-x)^2*(1+x^2)).
a(n) = -a(-n-1) = (42*n-6*i^(n*(n-1))-7*(-1)^n+5)/8 +2, where i=sqrt(-1).
MATHEMATICA
CoefficientList[Series[(1 + 7 x + 5 x^2 + 7 x^3 + x^4)/(1 - x - x^4 + x^5), {x, 0, 60}], x]
LinearRecurrence[{1, 0, 0, 1, -1}, {1, 8, 13, 20, 22}, 60] (* Vincenzo Librandi, Aug 18 2013 *)
PROG
(PARI) Vec((1+7*x+5*x^2+7*x^3+x^4)/(1-x-x^4+x^5)+O(x^60))
(Maxima) makelist(coeff(taylor((1+7*x+5*x^2+7*x^3+x^4)/(1-x-x^4+x^5), x, 0, n), x, n), n, 0, 60);
(Magma) I:=[1, 8, 13, 20, 22]; [n le 5 select I[n] else Self(n-1) +Self(n-4)-Self(n-5): n in [1..60]]; // Vincenzo Librandi, Aug 18 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Bruno Berselli, Nov 26 2012
STATUS
approved